PPFS-AG-SG-2<BR>




	<H1><H2<CENTER>WHEAT BACTERIAL STREAK</CENTER></H2></H1>



	<H4>by Don Hershman and Paul Bachi				(Revised 7-94)</H4>



	For the past few years, wheat scientists in the midsouth have been greatly concerned with a disease of wheat caused by the bacterium, Xanthomonas translucens f.sp. undulosa. When the bacterium affects leaf tissue, the resulting disease is known as bacterial streak. When it invades the head the disease is called black chaff. The disease has primarily been a problem in the lower midsouth; however, indications are, it is becoming a problem in more northern areas as well.



	Black chaff, which can easily be confused with a fungal disease, glume blotch, caused by Septoria nodorum, has been an increasing problem in wheat in Kentucky. However, the foliar phase, bacterial streak, has not yet become a serious problem. 



	While control measures for bacterial streak/black chaff are still being worked out, it is very important to be able to separate bacterial infection from diseases of similar appearance caused by fungi. The main reason for this is to ward off the possibility of a producer applying a fungicide to control a bacterial problem, which will have no effect on the disease. Also, the bacterium that causes bacterial streak/black chaff is seed-borne, so there are implications as far as saving seed for use next year.<BR>



<B>LEAF SYMPTOMS</B><BR>


	Infections start as small water-soaked spots usually on upper leaves. These spots enlarge, coalesce, become glossy, and eventually olive green translucent streaks of various lengths that later turn yellow-brown. Streaks may extend the length of a leaf and are usually somewhat narrow, being limited by leaf veins. Spots may become large and blotchy, causing the leaf to shrivel, often dying from the tips back. Infected areas tend to glisten in the sun, particularly under humid conditions.

	These symptoms may be confused with leaf blotches caused by various fungal pathogens. Generally, however, symptoms caused by fungi can be separated from those associated with bacterial streak, based on the presence of spore-producing structures and spores in the fungal incited lesions.<BR>



<B>HEAD SYMPTOMS</B><BR>


	Infected glumes will have dark, longitudinal streaks and spots that are more abundant on the upper parts of glumes. Beards of bearded varieties can also be affected. Grain is usually not destroyed, but it may be brown and shriveled. Yield effects are most pronounced when the bacteria invade the stem, usually just below the head and in upper joints.<BR>


<B>DISEASE DEVELOPMENT</B><BR>


	The bacterium causing bacterial streak/black chaff is seed-borne, and overwinters in residue, various weeds, and in field soil. The causal bacterium is spread within a field by splashing rain, plant-to-plant contact and insects. It cannot infect wheat tissue directly, so it must enter plants via natural openings in plant structures or in sites wounded by freeze damage or insect feeding. Free water is necessary for the bacterium to enter wheat tissue. As a result, symptoms of the disease usually appear after several days of damp, rainy weather.<BR>




<B>CONTROL</B><BR>


	Bacterial streak/black chaff is difficult to control. Best results can be obtained by using only certified seed and allowing 2 years between wheat crops in a field. Also, do not rotate wheat with barley as the bacterium will be maintained in the field at high levels. Current research has indicated varying degrees of susceptibility for different varieties. However, this information is not sufficiently developed to be used as a reliable control measure. The use of agronomic practices and varieties that minimize the chances of a field sustaining early spring freeze damage will help avoid serious damage due to bacterial streak/black chaff. Control insects where practical to reduce the frequency of sites available for the causal bacterium to move and enter plant tissue.<BR>